Table of Contents
What is Hot Working?
Hot-working is the process of deforming metal plastically above its recrystallization temperature, or below its melting point to improve its mechanical properties and grain structure. The hot working process helps to change the existing grain structure of the metal, and plastic deformation helps to give the desired shape and improve or forms new mechanical properties of the material.
Recrystallization temperature is different for different materials. It is necessary to maintain the temperature within the recrystallization range because high temperatures can burn the metal. Hot-working reduces metal deformation and increases resistance to crack under high forces.
The hot working process is carried out in steps. The working temperature is different is in each step, and it is above the minimum working temperature. In each step the temperature reduces, and in the final step, it is just above the minimum working temperature. Due to that, metals get heated several times to give shape and increase their ductility, and toughness.
In the final stage, the temperature is kept around minimum working temperature to achieve a good finish.
Softening in Hot Working
- Dynamic recovery- The activation energy in hot working is equal to that for creep and self-diffusion.
- Dynamic recrystallization- Activation energy for softening in hot working is higher than for creep.
Hot-working methods
- Hot rolling
- Hot forging
- Hot extrusion
- Hot piercing
- Hot drawing
- Hot spinning
Hot Rolling
Hot rolling is the process of metal forming in which a metal stack is heated above recrystallization temperature and passes through the pair or more pairs of identical rollers running at the same speed but in opposite directions. The purpose of hot rolling is to reduce the thickness, cross-sectional area, increase length, and obtain a uniform thickness of the metal stack.
This process changes the coarse grain structure of the metal into the fine grain structure. It helps to reduce internal stresses, defects, and porosity in metal.
Hot rolling improves the mechanical properties of the material, such as ductility, toughness, strength, weldability, formability, and vibration resistance.
Hot Forging
In this process, heated metal above the recrystallization temperature, or 75% of its melting temperature, is imposed by the external compressive force to give the desired shape. It uses a hammer or big press machine to apply a compressive force. During compression, if metal temperature reduces, then it is reheated. Due to the application of the compressive force, internal cracks get reduced.
This process is used for the manufacturing of crankshafts, crane hooks, and wrenches. It gives high ductility, high fatigue, good surface finish, and tensile strength to metal objects.
Hot Extrusion
Hot extrusion is the process of heating material above its recrystallization temperature pushed through a die of the desired cross-section to make the desired shape. The hydraulic or mechanical medium is used to apply pressure. Sharp corners are easy to achieve in this process. In mass production industries, this process is very useful. It is suitable for a brittle and ductile material.
The hot extrusion process is very useful to make complex parts. It reduces setup time, and tooling cost.
Hot piercing
Hot piercing is the process of making thin-walled seamless tubes. This process is also known as seamless tubing. In this process, a heated cylindrical billet passes over the two taper concave rollers set at an angle and rotates in the same direction. Due to the angle, these billet travels forward and rotate simultaneously. To make a seamless pipe, a tapered mandrel is inserted into the billet and causes the material to force outward. This material is compressed by the rollers over the mandrel and its length increase.
Hot drawing
In the hot drawing process, heated metal passes through the die, and tensile force is used to stretch it to make a sheet, bar, tube, or wire. This process reduces the size of the material. Dies of multiple shapes we can use to make different objects.
Hot spinning
In the hot spinning process, heat is applied with the torch on the workpiece spinning on the lathe machine. This workpiece is spinning over the rotating shape, and the desired shape is given with the tool.
Difference between hot working and cold working process
| Sr.No | Aspect | Hot Working Process | Cold Working Process |
|---|---|---|---|
| 1 | Definition | Hot-working is the process of deforming metal plastically above its recrystallization temperature and below its melting point to improve its mechanical properties like ductility. | Cold working is the process of strengthening metal plastically deformation below its recrystallization temperature. |
| 2 | Residual Stresses | No residual stresses formed in the metal. | Residual stresses form in the metal. |
| 3 | Mechanical properties | The grain structure of metal refines, and which leads to improvisation of mechanical properties. | There is a distortion of grain structure occurs, which leads to decreases in mechanical properties. |
| 4 | Stress | Less stress | More stress |
| 5 | Cracks | It removes cracks. | It forms new cracks. |
| 6 | Uniformity | High after the hot working process. | Low after cold working process. |
| 7 | Oxidation | Oxidations take place. | Oxidation does not take place. |
| 8 | Pickling | Require | Not require |
| 9 | Surface Finish | Achieve poor surface finish. | Achieve a better surface finish. |
| 10 | Close Dimensional Tolerance | Cannot achieve. | Can achieve. |
Advantages of Hot working process
- No strain hardening takes place because of heating metal above recrystallization temperature.
- Suitable in mass production industries.
- Easy to change the shape and size of the workpiece.
- It gives good ductility to the materials.
- It improves the grain structure of the metal.
- Stresses and Porosity in the materials reduces.
- Possible to achieve large deformation.
- Improves chemical, mechanical, and physical properties of the material.
- Cost-saving process.
- Fast and reliable process.
Disadvantages of Hot working process
- Lower surface finishes due to the formation of oxidation due to heat.
- Loss of carbon causes strength reduction of the outer surface.
- Reduction of strength creates fatigue cracks.
- High capital cost.
- Suitable for few materials.
- Dimensional accuracy is difficult to achieve.
- The maintenance cost is high.
Applications Hot working
- The hot working process is useful in making sheets, wires, tubes, pipes, etc
- Useful in automobile, heavy machinery, aerospace, defense, and shipping industries.